1. Field of the Invention
The present invention relates to a fixing apparatus for heating a recording material bearing an unfixed image to fix the unfixed image onto the recording material.
2. Description of the Related Art
A certain fixing apparatus mounted on an image forming apparatus, such as a copying machine and a printer, includes an endless film, a ceramic heater in contact with the inner surface of the endless film, and a pressing roller for forming a fixing nip portion with the ceramic heater via the endless film. When an image forming apparatus mounting this fixing apparatus performs continuous printing on small-size paper, a phenomenon of gradual temperature rise occurs at areas in a longitudinal direction of the fixing nip portion through which paper does not pass (this phenomenon is referred to as temperature rise at the sheet non-passing portions). If the temperature of the sheet non-passing portions rises too high, each part in the apparatus may be damaged. If printing is performed on large-size paper in a state of temperature rise at the sheet non-passing portions, a phenomenon in which toner at areas corresponding to the sheet non-passing portions for small-size paper is excessively heated and offset onto the film may arise (this phenomenon is referred to as high-temperature offsetting).
As a method for suppressing temperature rise at the sheet non-passing portions, a method for providing a ceramic heater with a member having thermal conduction anisotropy, represented by a graphite sheet is proposed (Japanese Patent Application Laid-Open No. 2003-317898 and Japanese Patent Application Laid-Open No. 2003-007435). Graphite has a structure in which hexagonal plate crystals composed of carbon are combined in layer form, and layers are combined by the Van der Waals' forces. Graphite provides high thermal conductivity in a direction parallel to the plane of the ceramic heater (in a direction parallel to the plane of covalent bond layers of graphite). Therefore, temperature rise at the sheet non-passing portions for small-size paper can be prevented by providing a graphite sheet on a ceramic substrate. Hereinafter, a member having thermal conduction anisotropy, such as a graphite sheet, is referred to as a heat leveling sheet. As discussed in Japanese Patent Application Laid-Open No. 2014-055104, a graphite sheet having high thermal conductivity in a direction parallel to the sheet plane is manufactured through heat processing on a polyimide film, which is a raw material.
The amount of heat transport of the heat leveling sheet in a planar direction of the heat leveling sheet(in a direction parallel to the sheet plane) can be obtained by multiplying the thermal conductivity in a planar direction of the heat leveling sheet by the thickness of the heat leveling sheet. To increase the amount of heat transport of a sheet to heighten the effect of suppressing temperature rise at the sheet non-passing portions, it is necessary to increase the thermal conductivity in a planar direction of the heat leveling sheet or to increase the thickness of the heat leveling sheet.
However, there have been the following problems that arise if a thick graphite sheet is to be disposed on the back side of the ceramic heater.
As a first problem, it is harder to manufacture a thick graphite sheet than to manufacture a thin graphite sheet while maintaining high thermal conductivity in a direction parallel to the sheet plane. Therefore, an effect of reducing temperature rise at the sheet non-passing portions by a thick graphite sheet is not so large as expected.
To manufacture a sheet having high thermal conductivity in a direction parallel to the sheet plane, a uniform molecular orientation is important. Processes for acquiring this characteristic include selecting a material having high molecular orientation from among polyimide films, which are raw materials, and applying a voltage to a graphite sheet in the manufacturing process. In this way, many processes are required to manufacture a thick graphite sheet. For this reason, many commercial graphite sheets having thermal conductivity exceeding 1000 W/(m·K) have a thickness of less than 100 μm.
As a second problem, the first printout time (FPOT) of an image forming apparatus is prolonged. The FPOT refers to a time period since a print signal is transmitted to a printer until the first sheet of recording material is discharged from the printer. To shorten the FPOT, it is necessary to use members having low heat capacity in the fixing apparatus. However, increasing the thickness of a graphite sheet increases the heat capacity of the sheet, resulting in an increase in heat capacity of the entire fixing apparatus. Further, the thermal conductivity of a graphite sheet in the thickness direction is sufficiently lower than that in a direction parallel to the sheet plane, but is higher than that of a heater holder which supports the ceramic heater. Therefore, the heat of the ceramic heater easily radiates to the heater holder, degrading the efficiency of heat supply to a recording material.